1. Field of the Invention
The present invention relates to a tire uniformity measuring apparatus for simply measuring variations in the lateral, radial and tangential loads acting on the tire being measured.
2. Description of the Prior Art
One of the factors which govern vibrations, noise and drivability of a vehicle is considered to be tire uniformity, that is, the uniformity in the outer shape, dimensions and rigidity of the tires. A variety of tire uniformity measuring apparatus have heretofore been proposed and put to practical use for checking such tire uniformity.
Conventional tire uniformity measuring apparatus include a rotatable cylindrical drum against which a tire being measured is held at a fixed interaxial distance for measuring a variation in the reactive force (hereinafter referred to as "R.F.V.") acting radially on the drum shaft or tire shaft while the drum and the tire are in rolling movement. With the fixed interaxial distance between the drum and the tire, the conventional apparatus are required to have a relatively high overall rigidity. The known apparatus are also disadvantageous in that the drum must be driven to rotate at high speeds, say, several tens of revolutions per minute, a sophisticated electrical measurement circuit is required and the apparatus are large in size, costly to construct and take a long period of time for measurement.
Under these circumstances, the present inventors have devised a tire uniformity measuring apparatus which eliminates the above-described problems of the conventional apparatus (see U.S. Pat. No. 4,458,526).
In this tire uniformity measuring apparatus, the interaxial distance between the tire that rolls under a certain load and the drum is not fixed, but a small displacement of the rotatable shaft to which the tire is secured is positively allowed to thereby detect a force corresponding to the reactive force acting on the shaft as a displacement of a movable member. The measuring apparatus can measure a variation in the radial force acting on the tire with high precision under a condition close to an actual running condition. The measuring apparatus is also easy to handle, light in weight, compact and less costly to construct.
However, when the tire has a large localized deformation, a large lateral force component is generated in the surface where the tire and the drum are held in contact with each other while they are in rolling movement, and a moment due to such lateral force component causes a large variation in the R.F.V. value. The resulting R.F.V. value must therefore be compensated for. Thus, the proposed prior art apparatus involves the problem of influence of such a lateral force component.
In view of the above, the present inventors have devised a tire uniformity measuring apparatus for accurately measuring a variation in the radial force acting on a tire while eliminating any influence of a moment due to a lateral force imposed on the tire under a condition close to an actual running condition.
However, the proposed measuring apparatus still suffers from the following problems. When the tire has a large localized deformation or when the tire has such a deformation that the tire rolling surface is not uniform in the longitudinal and lateral directions, variations in the lateral and tangential loads are generated in addition to a variation in the radial tire load. However, the proposed prior art apparatus cannot readily detect and evaluate these additional variations (hereinafter referred to as "L.F.V. and T.F.V.", respectively).